At present, three modes of filtering are known and used with inside-skin tubular membranes:
dead-end filtering, where all of the water injected into the tubes passes through the membranes and is collected as a permeate (filtered water) from the periphery of a bundle of membranes, with the flow rate at the longitudinal ends of the tubes being zero, and with the membranes being cleaned by periodic backwashing when the thickness of the layer of matter in suspension stopped by the membrane and deposited thereon gives rise to an increase in head loss which, at constant flow rate, increases pressure above an acceptable threshold, the backwashing removing said layer of deposited matter, and being triggered automatically by a controller;
cross-flow filtering where only a portion of the water injected into the tubes passes through the membranes and is collected as a permeate, while the remainder is collected at the axial outlet from the membranes and is recirculated in a loop, with the membranes being cleaned both by periodic backwashing and also on a continuous basis by the flow of water along the tubes, thereby enabling a portion of the matter in suspension to be kept in suspension in the flow instead of being deposited on the membranes, thus limiting the thickness of the cake; and
cross-flow filtering with continuous purging, making use of the above-described cross-flow filtering technique and where a portion of the recirculating water is purged in order to limit the concentration of matter in suspension in the circuit.
If there is a change in the turbidity or the organic matter content of the feed water beyond the threshold for good operation in dead-end mode, it would be advantageous to switch from one mode of filtering to another while using the same circuit. Circuits are generally designed either to operate in dead-end mode with water to be treated being inserted at one or other end or at both ends, or else to operate in cross-flow mode, with the recirculation pump imposing flow along the tubular membranes in one direction or the other depending on the position of the pump in the circuit, and with the pump feeding the tubular membranes from one end or the other. This applies not only to tubular membranes that are disposed horizontally, but also to tubular membranes that are disposed vertically, in which case the flow may be upwards or downwards with the membranes being fed respectively from the bottom or from the top.
However, FR-90/10 864 describes a method enabling both dead-end mode filtering and cross-flow mode filtering to be performed in the same circuit.
In any event, backwashing, which consists in injecting clean water into the modules containing the membranes in the opposite direction to that in which filtered water is collected, must ensure that the membranes are cleaned efficiently with limited consumption of energy and of clean water, and at time intervals which are large enough to be able to maintain adequate mean production of filtered water. The various different modes of backwashing that have been implemented in the past do not satisfy all of these conditions and they therefore need to be improved.